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Life cycle assessment applied to recycled aggregate concrete 213

allocation principle, for example, by mass value or by economic value. According
to Chen et al. (2010b) and Chen (2009), the mass allocation coefficients for fly ash,
blast-furnace slag and silica fume amount to 12.4%, 19.4% and 13%, respectively.
Given these considerable percentages, enormous environmental impacts are
imposed this way to the by-products which may discourage the concrete industry to
use them further on as a cement replacing material (Chen et al., 2010b). By addi-
tionally accounting for the obvious price difference between main product and by-
product, economic allocation coefficients are much lower, that is, 1.0%, 2.3% and
4.8%, respectively (Chen et al., 2010b; Chen, 2009). However, incorporation of the
price aspect in these percentages automatically implies that they are much less sta-
ble. Considerable variations are to be expected both in time and by country. For
instance, these values are representative for France in 2009 10. In Van den Heede
(2014), economic allocation coefficients were calculated for both fly ash and silica
fume used in Belgium in 2012 2014. They amounted to 2.9% and 6.1%, respec-
tively, and are thus considerably higher than the previous ones. Consequently, the
environmental impacts to be assigned to them will also be significantly higher. The
expected emissions to air of CO 2 ,SO x ,NO x and dust for fly ash, blast-furnace slag
and silica fumes using the mass and economic allocation coefficients of Chen et al.
(2010b) and Chen (2009) have been summarised in Table 9.3.
Clearly the choice of allocation principle has a huge impact on the emissions to
be assigned to each of the considered supplementary cementitious materials. When
looking at CO 2 , mass allocation for fly ash and silica fume implies higher emission
values than the corresponding ones for ordinary Portland cement. For fly ash and
silica fume, this problem also seems to exist for SO x and NO x emissions. Economic
allocation tends to result in a more fair emission contribution for all key emissions,
except in case of silica fume in terms of CO 2 emissions which is still 1.66 kg/kg.
Note that the choice between mass and economic allocation is still subject of a lot
of debate. The most robust and stable approach is mass allocation, yet it imposes
huge impacts to the supplementary cementitious materials. As this may discourage
the concrete industry to keep on using these materials as cement replacing material
and stimulate their landfilling again, it seems not the appropriate way of dealing
with these by-products. However, economic allocation holds the major disadvantage
of being highly dependent on price variations in time and by country. However, the

Table 9.3 CO 2 ,SO x ,NO x and dust emissions to be assigned to 1 kg of fly ash, blast-
furnace slag and silica fume using the mass and economic allocation coefficients Chen
et al. (2010b) and Chen (2009)

1 kg Fly ash Blast-furnace slag Silica fume
Allocation Mass Economic Mass Economic Mass Economic
CO 2 (kg) 2.27 0.18 0.69 0.08 4.51 1.66
SO x (kg) 1.06 3 10 22 0.86 3 10 23 1.08 3 10 24 0.13 3 10 24 1.07 3 10 22 0.39 3 10 22
NO x (kg) 4.67 3 10 23 0.38 3 10 23 6.45 3 10 25 0.76 3 10 25 8.51 3 10 23 3.14 3 10 23
Dust (kg) 8.06 3 10 24 0.65 3 10 24 2.59 3 10 25 0.31 3 10 25 6.77 3 10 23 2.50 3 10 23